This invention relates to a method of and apparatus for monitoring the redox state of elements in glass.
The redox state of variable valency elements in glass is governed by the conditions under which the glass is formed, the principal determining factors being the composition of the atmosphere under which the glass is melted and the composition of the batch. Thus the redox state of such elements is governed by the fuel/air mixture feeding the flames of a glass melting furnace, and by the quantities of oxidising agents such as sodium sulphate and of reducing agents such as sulphides, coke and metallic ions which may be incorporated into the batch feeding the furnace. Other factors also have an effect on the redox state of elements in the glass, and among these factors may be cited the temperatures to which the molten glass is subjected during manufacture, and the time for which the glass is subjected to such temperatures. The redox state can thus vary with the rate of glass output from the furnace, whether this can be a gas-fired furnace or an electric furnace or a furnace which is heated by both gas and electricity. The redox state can also vary with the state of the checker-worker in a regenerator furnace, and thus with the age of the furnace.
The redox state of variable valency elements in glass can have an important effect on the properties, particularly the radiation transmitting properties, of the glass produced. By way of example, the following may be cited:
Iron. Iron is present in almost all commercially produced glass. either as an impurity, or as a deliberately introduced colouring agent. The presence of Fe.sup.3+ ions in glass gives rise to slight absorption of short-wavelength visible light and to a very strong absorption band in the ultra-violet region, while the presence of Fe.sup.2+ ions gives rise to strong absorption in the infra-red. Thus, for example, if it is desired to produce a glass having a high energy transmission in respect of solar radiaton, the iron should be in the higher oxidation state. In addition, ferric iron imparts a mild yellow colouration to the glass, and ferrous iron a stronger greenish-blue colour.
Sulphur. Sulphur is also present in much commercially produced glass, having been introduced as sulphate, as a refining agent, or sulphide, as a melting accelerator. While sulphur in higher oxidation states has practically no colouring effect, the S.sup.2- anion, especially in the presence of iron, can give rise to a yellowish-brown colour.
In addition to iron and sulphur, glass often contains other colouring agents, whether introduced deliberately or present as impurities. Some of the more important colouring agents for glass are referred to below.
Selenium. The Se.sup.4+ cation has practically no colouring effect, whereas the uncharged Se.sup.0 element imparts a pink colouration. The Se.sup.2- anion forms a chromophore with any ferric ions present, and this gives a brownish red colour to glass.
Chromium. The presence of the coordinate [Cr.sup.III O.sub.6 ] gives rise to absorption bands at 450 nm and 650 nm to give a clear green colour. Strong oxidation gives rise to the coordinate [Cr.sup.VI O.sub.4 ] which gives a very intense absorption band at 365 nm giving a yellow colouration.
Manganese. Mn.sup.2+ ions have practically no colouring effect, but Mn.sup.3+ ions give rise to a violet colour.
Nickel. The group [Ni.sup.II O.sub.4 ] gives rise to a blue colouration of the glass, and the group [Ni.sup.II O.sub.6 ] to a yellow colouration.
The colouring effect of other agents such as cobalt, cerium, copper, titanium and vanadium is also dependent on their oxidation state.
The importance of monitoring the redox of the glass constituents in order to control the quality of the glass produced will therefore be appreciated.
Hitherto, the redox state of the glass constituents has been monitored in respect of the glass after it has been formed into the desired product, whether this be sheet glass or hollow ware. Such monitoring was effected indirectly by optical spectroscopy, X-ray fluorescence, and electronic paramagnetic resonance techniques. By way of example, for clear glass containing iron, the relative proportions of ferric and ferrous ions present was calculated from the transmissivity of the glass in respect of light having wavelengths of 380 nm and 1050 nm, and the total iron concentration was obtained by X-ray fluorescence.